Count the number of rafters needed for the roof. If the roof has a basic design, such as a gable or flat roof, the rafters should be the same length on the entire roof. If the roof has a multi-surface design, such as a hip roof or gambrel roof, break the rafters into different groups based on the length, and count how many rafters are needed at each length.
Multiply the length of the rafters by how many rafters you need. If the rafters are different lengths, add up the measurements from each length.
Add up the total length of any beams in the roof design, just as you did with the rafters. The number of beams, and their length, can vary depending on the roof design.
Add the length of the the rafters and beams together, and divide it by the length of the 2-by-6 or 2-by-8 boards you're using to build the roof frame to determine how many boards you need.
Multiply the length and width of the roof together to determine the square footage that must be covered by the plywood or oriented-strand board (OSB). If the roof is oddly-shaped, break it down into smaller sections, then add the square footage of the different sections together.